In a known manner, there are memorized in the known pacemakers intended for controlling the heart, two values of frequencies for the stimulation of the ventricle. One value is the basic frequency or the minimum frequency of stimulation, and the other value is the maximum frequency beyond which it is hazardous to stimulate the ventricle. When the pacemaker detects an increase in atrial activity, it stimulates the ventricle at a frequency approaching the atrium frequency, however, without exceeding the maximum set frequency.
The first problem which arises in known cardiac pacemakers is the difficulty in distinguishing a physiological acceleration of the atrial rhythm from a pathological acceleration.
A second problem which arises in presence of an acceleration of the atrium beyond the maximum frequency is to appreciate the duration during which the heart is stimulated at a fast frequency, before switching to slow-down or fall back mode towards a basic frequency.
The third problem during this slowing-down procedure is the dissociation between depolarization at the atrial stage and depolarization at the ventricular stage.
In existing pacemakers, these problems are solved in an imperfect manner. U.S. Pat. No. 4,467,810, for instance describes a pacemaker in which the stimulation procedure in the known 2/1 mode (a ratio of two atrial depolarizations to one ventricular depolarization) takes place as soon as the depolarization frequency of the atrium reaches the reference frequency, and depolarizations of the atrium which occur at a frequency exceeding a programmed reference frequency are systematically ignored. Besides this, the described pacemaker uses a fall-back procedure, that is, a procedure for slowing down the rhythm of stimulation of the ventricle, independently of the atrium rhythm.
There is, however, a continuing need for improved pacemaker devices and methods of cardiac activity signal processing for controlling cardiac rhythm particularly in the presence of atrial extrasystoles.